Planetary gear train of automatic transmission for vehicle

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque of an engine, an output shaft outputting changed torque, a first planetary gear set, a second planetary gear set, a third planetary gear set, a fourth planetary gear set, a first rotation shaft, a second rotation shaft directly connected to the input shaft, a third rotation shaft, a fourth rotation shaft selectively connected to the first rotation shaft or the second rotation shaft, a fifth rotation shaft selectively connected to a transmission housing, a sixth rotation shaft selectively connected to the transmission housing, a seventh rotation shaft selectively connected to the first rotation shaft, and an eighth rotation shaft directly connected to the output shaft, and selectively connected to the first rotation shaft.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2014-0178381 filed Dec. 11, 2014, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission of a vehicle which improvespower delivery performance and reduces fuel consumption by achieving tenforward speed stages using a minimum number of constituent elements.

2. Description of Related Art

In recent years, a rise in oil price causes a rise in competition fortechnologies to enhance fuel efficiency.

As a result, research into reduction of weight and enhancement of fuelefficiency through down-sizing is being conducted in the case of anengine and research for simultaneously securing operability and fuelefficiency competitiveness through multiple speed stages is beingconducted in the case of an automatic transmission.

However, in the automatic transmission, as the number of speed stagesincrease, the number of internal components increase, and as a result,mountability, transmission efficiency, and the like may stilldeteriorate and cost, and weight may increase.

Accordingly, development of a planetary gear train which may bring aboutmaximum efficiency with as small a number of components may be importantin order to increase a fuel efficiency enhancement effect through themulti-stages.

In this aspect, in recent years, 8 and 9-speed automatic transmissionstend to be implemented and the research and development of a planetarygear train capable of implementing more speed stages has also beenactively conducted.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission of a vehicle havingadvantages of improving power delivery performance and fuel economy byachieving ten forward speed stages and one reverse speed stage usingminimum number of constituent elements and of improving silent drivingusing a driving point positioned at a low engine speed.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft receiving torque of an engine, an output shaft outputting changedtorque, a first planetary gear set including a first sun gear, a firstplanet carrier, and a first ring gear, a second planetary gear setincluding a second sun gear, a second planet carrier, and a second ringgear, a third planetary gear set including a third sun gear, a thirdplanet carrier, and a third ring gear, a fourth planetary gear setincluding a fourth sun gear, a fourth planet carrier, and a fourth ringgear, a first rotation shaft including the first sun gear, a secondrotation shaft including the first planet carrier and directly connectedto the input shaft, a third rotation shaft including the first ringgear, the second planet carrier, and the third ring gear, a fourthrotation shaft including the second and third sun gears and selectivelyconnected to the first rotation shaft or the second rotation shaft, afifth rotation shaft including the third planet carrier and the fourthring gear and selectively connected to a transmission housing, a sixthrotation shaft including the second ring gear and selectively connectedto the transmission housing, a seventh rotation shaft including thefourth sun gear and selectively connected to the first rotation shaft,and an eighth rotation shaft including the fourth planet carrier,directly connected to the output shaft, and selectively connected to thefirst rotation shaft.

Each of the first, second, third, and fourth planetary gear sets mayinclude a single pinion planetary gear set.

The first, second, third, and fourth planetary gear sets may be disposedsequentially from an engine side.

The planetary gear train may further include a first clutch interposedbetween the second rotation shaft and the fourth rotation shaft, asecond clutch interposed between the first rotation shaft and the eighthrotation shaft, a third clutch interposed between the first rotationshaft and the fourth rotation shaft, a fourth clutch interposed betweenthe first rotation shaft and the seventh rotation shaft, a first brakeinterposed between the sixth rotation shaft and the transmissionhousing, and a second brake interposed between the fifth rotation shaftand the transmission housing.

A first forward speed stage may be achieved by operation of the fourthclutch and the first and second brakes, a second forward speed stage maybe achieved by operation of the first and fourth clutches and the secondbrake, a third forward speed stage may be achieved by operation of thefirst and fourth clutches and the first brake, a fourth forward speedstage may be achieved by operation of the first, third, and fourthclutches, a fifth forward speed stage may be achieved by operation ofthe third and fourth clutches and the first brake, a sixth forward speedstage may be achieved by operation of the second and fourth clutches andthe first brake, a seventh forward speed stage may be achieved byoperation of the second and third clutches and the first brake, aneighth forward speed stage may be achieved by operation of the first andsecond clutches and the first brake, a ninth forward speed stage may beachieved by operation of the second clutch and the first and secondbrakes, a tenth forward speed stage may be achieved by operation of thefirst and second clutches and the second brake, and a reverse speedstage may be achieved by operation of the third and fourth clutches andthe second brake.

According to various aspects of the present invention, a planetary geartrain of an automatic transmission for a vehicle may include an inputshaft receiving torque of an engine, an output shaft outputting changedtorque, a first planetary gear set including a first sun gear, a firstplanet carrier, and a first ring gear, a second planetary gear setincluding a second sun gear, a second planet carrier, and a second ringgear, a third planetary gear set including a third sun gear, a thirdplanet carrier, and a third ring gear, and a fourth planetary gear setincluding a fourth sun gear, a fourth planet carrier, and a fourth ringgear, in which the input shaft may be directly connected to the firstplanet carrier, the output shaft may be directly connected to the fourthplanet carrier, the first ring gear, the second planet carrier, and thethird ring gear may be directly connected to each other, the second sungear may be directly connected to the third sun gear, the third planetcarrier may be directly connected to the fourth ring gear, the firstplanet carrier may be selectively connected to the second sun gear, thefirst sun gear may be selectively connected to the fourth planetcarrier, the first sun gear may be selectively connected to the secondsun gear, the first sun gear may be selectively connected to the fourthsun gear, the second ring gear may be selectively connected to atransmission housing, and the fourth ring gear may be selectivelyconnected to the transmission housing.

The planetary gear train may further include a first clutch selectivelyconnecting the first planet carrier to the second sun gear, a secondclutch selectively connecting the first sun gear to the fourth planetcarrier, a third clutch selectively connecting the first sun gear to thesecond sun gear, a fourth clutch selectively connecting the first sungear to the fourth sun gear, a first brake selectively connecting thesecond ring gear to the transmission housing, and a second brakeselectively connecting the fourth ring gear to the transmission housing.

Various embodiments of the present invention may achieve ten forwardspeed stages by combining four planetary gear sets with six frictionelements. Therefore, power delivery performance and fuel economy may beimproved.

Since a speed stage suitable to engine speed can be achieved due tomultiple speed stages, silent driving may be improved.

It is understood that the term “vehicle” or “vehicular” or other similarterms as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuel derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example, bothgasoline-powered and electric-powered vehicles.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary planetary gear trainaccording to the present invention.

FIG. 2 is an operation chart of friction elements at each speed in theexemplary planetary gear train according to the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

FIG. 1 is a schematic diagram of a planetary gear train according tovarious embodiments of the present invention.

Referring to FIG. 1, a planetary gear train according to variousembodiments of the present invention includes first, second, third, andfourth planetary gear sets PG1, PG2, PG3, and PG4 disposed on the sameaxis, an input shaft IS, an output shaft OS, eight rotation shafts TM1to TM8 including at least one of rotation elements of the first, second,third, and fourth planetary gear sets PG1, PG2, PG3, and PG4, sixfriction elements C1 to C4 and B1 to B2, and a transmission housing H.

As a result, torque input from the input shaft IS is changed bycooperation of the first, second, third, and fourth planetary gear setsPG1, PG2, PG3, and PG4, and the changed torque is output through theoutput shaft OS.

The simple planetary gear sets are disposed in a sequence of the first,second, third, and fourth planetary gear sets PG1, PG2, PG3, and PG4from an engine side.

The input shaft IS is an input member and power from a crankshaft of anengine is torque-converted through a torque converter to be input intothe input shaft IS.

The output shaft OS is an output member, is disposed in parallel withthe input shaft IS, and transmits driving torque to a driving wheelthrough a differential apparatus.

The first planetary gear set PG1 is a single pinion planetary gear setand includes a first sun gear 51, a first planet carrier PC1 rotatablysupporting a first pinion P1 that is externally meshed with the firstsun gear S1, and a first ring gear R1 that is internally meshed with thefirst pinion P1 as rotation elements thereof.

The second planetary gear set PG2 is a single pinion planetary gear setand includes a second sun gear S2, a second planet carrier PC2 rotatablysupporting a second pinion P2 that is externally meshed with the secondsun gear S2, and a second ring gear R2 that is internally meshed withthe second pinion P2 as rotation elements thereof.

The third planetary gear set PG3 is a single pinion planetary gear setand includes a third sun gear S3, a third planet carrier PC3 rotatablysupporting a third pinion P3 that is externally meshed with the thirdsun gear S3, and a third ring gear R3 that is internally meshed with thethird pinion P3 as rotation elements thereof.

The fourth planetary gear set PG4 is a single pinion planetary gear setand includes a fourth sun gear S4, a fourth planet carrier PC4 rotatablysupporting a fourth pinion P4 that is externally meshed with the fourthsun gear S4, and a fourth ring gear R4 that is internally meshed withthe fourth pinion P4 as rotation elements thereof.

In the first, second, third, and fourth planetary gear sets PG1, PG2,PG3, and PG4, one or more rotation elements are directly connected toeach other to operate with eight rotation shafts TM1 to TM8.

The eight rotation shafts TM1 to TM8 will be described in furtherdetail.

The first rotation shaft TM1 includes the first sun gear S1.

The second rotation shaft TM2 includes the first planet carrier PC1 andis directly connected to the input shaft IS so as to be always operatedas an input element.

The third rotation shaft TM3 includes the first ring gear R1, the secondplanet carrier PC2, and the third ring gear R3.

The fourth rotation shaft TM4 includes the second and third sun gears S2and S3, and is selectively connected to the first rotation shaft TM1 orthe second rotation shaft TM2.

The fifth rotation shaft TM5 includes the third planet carrier PC3 andthe fourth ring gear R4 and is selectively connected to the transmissionhousing H.

The sixth rotation shaft TM6 includes the second ring gear R2 and isselectively connected to the transmission housing H.

The seventh rotation shaft TM7 includes the fourth sun gear S4 and isselectively connected to the first rotation shaft TM1.

The eighth rotation shaft TM8 includes the fourth planet carrier PC4, isdirectly connected to the output shaft OS, and is selectively connectedto the first rotation shaft TM1.

In addition, four clutches C1, C2, C3, and C4 which are frictionelements are disposed at connection portions between the input shaft ISand any one rotation shaft or between any two rotation shafts.

In addition, two brakes B1 and B2 which are friction elements aredisposed at connection portions between any one rotation shaft and thetransmission housing H.

The six frictional elements C1 to C4 and B1 to B2 will be described infurther detail.

The first clutch C1 is interposed between the second rotation shaft TM2and the fourth rotation shaft TM4 and selectively connects the secondrotation shaft TM2 to the fourth rotation shaft TM4.

The second clutch C2 is interposed between the first rotation shaft TM1and the eighth rotation shaft TM8 and selectively connects the firstrotation shaft TM1 to the eighth rotation shaft TM8.

The third clutch C3 is interposed between the first rotation shaft TM1and the fourth rotation shaft TM4 and selectively connects the firstrotation shaft TM1 to the fourth rotation shaft TM4.

The fourth clutch C4 is interposed between the first rotation shaft TM1and the seventh rotation shaft TM7 and selectively connects the firstrotation shaft TM1 to the seventh rotation shaft TM7.

The first brake B1 is interposed between the sixth rotation shaft TM6and the transmission housing H and causes the sixth rotation shaft TM6to be operated as a fixed element.

The second brake B2 is interposed between the fifth rotation shaft TM5and the transmission housing H and causes the fifth rotation shaft TM5to be operated as a fixed element.

The friction elements including the first, second, third, and fourthclutches C1, C2, C3, and C4 and the first and second brakes B1 and B2may be multi-plates friction elements of wet type that are operated byhydraulic pressure.

FIG. 2 is an operation chart of friction elements at each speed in theplanetary gear train according to various embodiments of the presentinvention.

As shown in FIG. 2, three friction elements are operated at each speedstage in the planetary gear train according to various embodiments ofthe present invention.

The fourth clutch C4 and the first and second brakes B1 and B2 areoperated at a first forward speed stage 1ST.

The first and fourth clutches C1 and C4 and the second brake B2 areoperated at a second forward speed stage 2ND.

The first and fourth clutches C1 and C4 and the first brake B1 areoperated at a third forward speed stage 3RD.

The first, third, and fourth clutches C1, C3, and C4 are operated at afourth forward speed stage 4TH.

The third and fourth clutches C3 and C4 and the first brake B1 areoperated at a fifth forward speed stage 5TH.

The second and fourth clutches C2 and C4 and the first brake B1 areoperated at a sixth forward speed stage 6TH.

The second and third clutches C2 and C3 and the first brake B1 areoperated at a seventh forward speed stage 7TH.

The first and second clutches C1 and C2 and the first brake B1 areoperated at an eighth forward speed stage 8TH.

The second clutch C2 and the first and second brakes B1 and B2 areoperated at a ninth forward speed stage 9TH.

The first and second clutches C1 and C2 and the second brake B2 areoperated at a tenth forward speed stage 10TH.

The third and fourth clutches C3 and C4 and the second brake B2 areoperated at a reverse speed stage REV.

The shifting processes will be described in further detail.

The fourth clutch C4 and the first and second brakes B1 and B2 areoperated at the first forward speed stage 1ST. In a state that the firstrotation shaft TM1 is connected to the seventh rotation shaft TM7 byoperation of the fourth clutch C4, rotation speed of the input shaft ISis input to the second rotation shaft TM2 and the fifth and sixthrotation shafts TM5 and TM6 are operated as the fixed elements byoperation of the second brake B2 and the first brake B1. Therefore, thefirst forward speed stage is achieved.

The first and fourth clutches C1 and C4 and the second brake B2 areoperated at the second forward speed stage 2ND. In a state that thefirst rotation shaft TM1 is connected to the seventh rotation shaft TM7by operation of the fourth clutch C4 and the second rotation shaft TM2is connected to the fourth rotation shaft TM4 by operation of the firstclutch C1, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the fifth rotation shaft TM5 is operatedas the fixed element by operation of the second brake B2. Therefore, thesecond forward speed stage is achieved.

The first and fourth clutches C1 and C4 and the first brake B1 areoperated at the third forward speed stage 3RD. In a state that the firstrotation shaft TM1 is connected to the seventh rotation shaft TM7 byoperation of the fourth clutch C4 and the second rotation shaft TM2 isconnected to the fourth rotation shaft TM4 by operation of the firstclutch C1, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the sixth rotation shaft TM6 is operatedas the fixed element by operation of the first brake B1. Therefore, thethird forward speed stage is achieved.

The first, third, and fourth clutches C1, C3, and C4 are operated at thefourth forward speed stage 4TH. In a state that the second rotationshaft TM2 is connected to the fourth rotation shaft TM4 by operation ofthe first clutch C1, the first rotation shaft TM1 is connected to thefourth rotation shaft TM4 by operation of the third clutch C3, and thefirst rotation shaft TM1 is connected to the seventh rotation shaft TM7by operation of the fourth clutch C4, the rotation speed of the inputshaft IS is input to the second rotation shaft TM2. Therefore, thefirst, second, third, and fourth planetary gear sets PG1, PG2, PG3, andPG4 become direct-coupling states, and the fourth forward speed stagewhere the rotation speed of the input shaft IS is just output isachieved.

The third and fourth clutches C3 and C4 and the first brake B1 areoperated at the fifth forward speed stage 5TH. In a state that the firstrotation shaft TM1 is connected to the fourth rotation shaft TM4 byoperation of the third clutch C3 and the first rotation shaft TM1 isconnected to the seventh rotation shaft TM7 by operation of the fourthclutch C4, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the sixth rotation shaft TM6 is operatedas the fixed element by operation of the first brake B1. Therefore, thefifth forward speed stage is achieved.

The second and fourth clutches C2 and C4 and the first brake B1 areoperated at the sixth forward speed stage 6TH. In a state that the firstrotation shaft TM1 is connected to the eighth rotation shaft TM8 byoperation of the second clutch C2 and the first rotation shaft TM1 isconnected to the seventh rotation shaft TM7 by operation of the fourthclutch C4, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the sixth rotation shaft TM6 is operatedas the fixed element by operation of the first brake B1. Therefore, thesixth forward speed stage is achieved.

The second and third clutches C2 and C3 and the first brake B1 areoperated at the seventh forward speed stage 7TH. In a state that thefirst rotation shaft TM1 is connected to the eighth rotation shaft TM8by operation of the second clutch C2 and the first rotation shaft TM1 isconnected to the fourth rotation shaft TM4 by operation of the thirdclutch C3, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the sixth rotation shaft TM6 is operatedas the fixed element by operation of the first brake B1. Therefore, theseventh forward speed stage is achieved.

The first and second clutches C1 and C2 and the first brake B1 areoperated at the eighth forward speed stage 8TH. In a state that thesecond rotation shaft TM2 is connected to the fourth rotation shaft TM4by operation of the first clutch C1 and the first rotation shaft TM1 isconnected to the eighth rotation shaft TM8 by operation of the secondclutch C2, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the sixth rotation shaft TM6 is operatedas the fixed element by operation of the first brake B1. Therefore, theeighth forward speed stage is achieved.

The second clutch C2 and the first and second brakes B1 and B2 areoperated at the ninth forward speed stage 9TH. In a state that the firstrotation shaft TM1 is connected to the eighth rotation shaft TM8 byoperation of the second clutch C2, the rotation speed of the input shaftIS is input to the second rotation shaft TM2 and the fifth and sixthrotation shafts TM5 and TM6 are operated as the fixed elements byoperation of the second brake B2 and the first brake B1. Therefore, theninth forward speed stage is achieved.

The first and second clutches C1 and C2 and the second brake B2 areoperated at the tenth forward speed stage 10TH. In a state that thesecond rotation shaft TM2 is connected to the fourth rotation shaft TM4by operation of the first clutch C1 and the first rotation shaft TM1 isconnected to the eighth rotation shaft TM8 by operation of the secondclutch C2, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the fifth rotation shaft TM5 is operatedas the fixed element by operation of the second brake B2. Therefore, thetenth forward speed stage is achieved.

The third and fourth clutches C3 and C4 and the second brake B2 areoperated at the reverse speed stage REV. In a state that the firstrotation shaft TM1 is connected to the fourth rotation shaft TM4 byoperation of the first clutch C1 and the first rotation shaft TM1 isconnected to the seventh rotation shaft TM7 by operation of the fourthclutch C4, the rotation speed of the input shaft IS is input to thesecond rotation shaft TM2 and the fifth rotation shaft TM5 is operatedas the fixed element by operation of the second brake B2. Therefore, thereverse speed stage is achieved.

The planetary gear train according to various embodiments of the presentinvention may achieve ten forward speed stages and one reverse speedstage by control of four planetary gear sets PG1, PG2, PG3, and PG4,four clutches C1, C2, C3, and C4, and two brakes B1 and B2.

Therefore, power delivery performance and fuel economy may be improved.

Since a speed stage suitable to engine speed can be achieved due tomultiple speed stages, silent driving may be improved.

In addition, since three friction elements operate at each speed and thenumber of non-operating friction elements is minimized, a friction dragloss is decreased. Therefore, drag torque and power loss may be reduced.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft receiving torque of an engine; an output shaft outputting changed torque; a first planetary gear set including a first sun gear, a first planet carrier, and a first ring gear; a second planetary gear set including a second sun gear, a second planet carrier, and a second ring gear; a third planetary gear set including a third sun gear, a third planet carrier, and a third ring gear; a fourth planetary gear set including a fourth sun gear, a fourth planet carrier, and a fourth ring gear; a first rotation shaft including the first sun gear; a second rotation shaft including the first planet carrier and directly connected to the input shaft; a third rotation shaft including the first ring gear, the second planet carrier, and the third ring gear; a fourth rotation shaft including the second and third sun gears and selectively connected to the first rotation shaft or the second rotation shaft; a fifth rotation shaft including the third planet carrier and the fourth ring gear and selectively connected to a transmission housing; a sixth rotation shaft including the second ring gear and selectively connected to the transmission housing; a seventh rotation shaft including the fourth sun gear and selectively connected to the first rotation shaft; and an eighth rotation shaft including the fourth planet carrier, directly connected to the output shaft, and selectively connected to the first rotation shaft.
 2. The planetary gear train of claim 1, wherein each of the first, second, third, and fourth planetary gear sets comprises a single pinion planetary gear set.
 3. The planetary gear train of claim 1, wherein the first, second, third, and fourth planetary gear sets are disposed sequentially from an engine side.
 4. The planetary gear train of claim 1, further comprising: a first clutch interposed between the second rotation shaft and the fourth rotation shaft; a second clutch interposed between the first rotation shaft and the eighth rotation shaft; a third clutch interposed between the first rotation shaft and the fourth rotation shaft; a fourth clutch interposed between the first rotation shaft and the seventh rotation shaft; a first brake interposed between the sixth rotation shaft and the transmission housing; and a second brake interposed between the fifth rotation shaft and the transmission housing.
 5. The planetary gear train of claim 4, wherein a first forward speed stage is achieved by operation of the fourth clutch and the first and second brakes, a second forward speed stage is achieved by operation of the first and fourth clutches and the second brake, a third forward speed stage is achieved by operation of the first and fourth clutches and the first brake, a fourth forward speed stage is achieved by operation of the first, third, and fourth clutches, a fifth forward speed stage is achieved by operation of the third and fourth clutches and the first brake, a sixth forward speed stage is achieved by operation of the second and fourth clutches and the first brake, a seventh forward speed stage is achieved by operation of the second and third clutches and the first brake, an eighth forward speed stage is achieved by operation of the first and second clutches and the first brake, a ninth forward speed stage is achieved by operation of the second clutch and the first and second brakes, a tenth forward speed stage is achieved by operation of the first and second clutches and the second brake, and a reverse speed stage is achieved by operation of the third and fourth clutches and the second brake.
 6. A planetary gear train of an automatic transmission for a vehicle, comprising: an input shaft receiving torque of an engine; an output shaft outputting changed torque; a first planetary gear set including a first sun gear, a first planet carrier, and a first ring gear; a second planetary gear set including a second sun gear, a second planet carrier, and a second ring gear; a third planetary gear set including a third sun gear, a third planet carrier, and a third ring gear; and a fourth planetary gear set including a fourth sun gear, a fourth planet carrier, and a fourth ring gear, wherein the input shaft is directly connected to the first planet carrier, the output shaft is directly connected to the fourth planet carrier, the first ring gear, the second planet carrier, and the third ring gear are directly connected to each other, the second sun gear is directly connected to the third sun gear, the third planet carrier is directly connected to the fourth ring gear, the first planet carrier is selectively connected to the second sun gear, the first sun gear is selectively connected to the fourth planet carrier, the first sun gear is selectively connected to the second sun gear, the first sun gear is selectively connected to the fourth sun gear, the second ring gear is selectively connected to a transmission housing, and the fourth ring gear is selectively connected to the transmission housing.
 7. The planetary gear train of claim 6, wherein each of the first, second, third, and fourth planetary gear sets comprises a single pinion planetary gear set.
 8. The planetary gear train of claim 6, wherein the first, second, third, and fourth planetary gear sets are disposed sequentially from an engine side.
 9. The planetary gear train of claim 6, further comprising: a first clutch selectively connecting the first planet carrier to the second sun gear; a second clutch selectively connecting the first sun gear to the fourth planet carrier; a third clutch selectively connecting the first sun gear to the second sun gear; a fourth clutch selectively connecting the first sun gear to the fourth sun gear; a first brake selectively connecting the second ring gear to the transmission housing; and a second brake selectively connecting the fourth ring gear to the transmission housing.
 10. The planetary gear train of claim 9, wherein a first forward speed stage is achieved by operation of the fourth clutch and the first and second brakes, a second forward speed stage is achieved by operation of the first and fourth clutches and the second brake, a third forward speed stage is achieved by operation of the first and fourth clutches and the first brake, a fourth forward speed stage is achieved by operation of the first, third, and fourth clutches, a fifth forward speed stage is achieved by operation of the third and fourth clutches and the first brake, a sixth forward speed stage is achieved by operation of the second and fourth clutches and the first brake, a seventh forward speed stage is achieved by operation of the second and third clutches and the first brake, an eighth forward speed stage is achieved by operation of the first and second clutches and the first brake, a ninth forward speed stage is achieved by operation of the second clutch and the first and second brakes, a tenth forward speed stage is achieved by operation of the first and second clutches and the second brake, and a reverse speed stage is achieved by operation of the third and fourth clutches and the second brake. 